1. Field of the Invention
The present invention relates to an optical disk comprising a printed layer on a signal readout surface side of the disk wherein the printed layer is used for displaying material such as the content of the recorded information, and in particular relates to technology which can reduce the effect of the printed layer on the electrical characteristics and stabilize the disk shape.
2. Description of the Related Art
In conventional optical disks, typically a printed layer is provided for displaying visual information such as figures and symbols so that the content of the recorded information can be readily understood.
For example, in the case of single sided recording reproduction type optical disks such as CDs (compact disks), a printed layer, which is formed by methods such as screen printing or offset printing, is provided onto the protective layer positioned on the disk surface reverse to the information signal readout surface, and hence almost the entire surface of the disk can be used for displaying figures and symbols.
In contrast, in the case of double sided recording reproduction type optical disks, onto which larger volumes of information are able to be recorded, the regions of the disk on which a printed layer can be provided are limited to those portions outside of the information recording regions such as the very restricted region close to the central portion of the disk, so as not to obstruct the readout of the information signal. Consequently the surface area which can be used for the display of figures and symbols has been very restricted.
In order to resolve the above problem, attempts have been made to enlarge the display area for visible information on double sided recording reproduction type optical disks, and examples include the inventions disclosed in Japanese Unexamined Patent Publication No. 981964, Japanese Unexamined Patent Publication No. 6-55887, and Japanese Unexamined Patent Publication No. 8-273201.
The inventions above comprise two optical disk base bodies, each of which is formed by sequentially laminating an information signal layer of a translucent reflective film, a protective layer, and a printed layer onto an optical disk substrate, and which are then bonded together with the printed layers facing each other. Consequently, the information signal layer has a predetermined transparency, and the printed layer is visible from the disk substrate side of each disk base body which functions as the information signal readout surface, through the information signal layer and the protective layer.
With such a construction, a large surface area for the printing and display of figures and symbols can be ensured on both sides of the disk, without obstructing the readout of the information signal, even with double sided recording reproduction type optical disks.
However with the conventional technology described above, because the protective layer is formed by the curing of an applied resin, there is a danger that contraction during curing will cause a large distortion of the disk base body, generating a curvature in the substrate. This curvature has become a large problem, particularly in those cases where a double sided recording reproduction type optical disk is produced by bonding two optical disk substrates together. In the aforementioned Japanese Unexamined Patent Publication No. 9-81964, the thickness of the protective layer was reported as being of the order of 40 xcexcm, but an application of a protective layer of a physical thickness of 40 xcexcm generates a considerable distortion in the shape of the disk, arising a problem in practical use.
Furthermore, light will reach the printed layer passing through the information signal layer of the translucent reflective film and the protective layer, and then undergo reflection and return along a reverse path to the signal readout surface of the disk. Therefore a problem arises in that the returned light (scattered and reflected light) is superimposed as a disturbance on the readout signal, lowering the electrical characteristics (jitter) of the optical disk.
The present invention takes the above factors into consideration, with an object of providing an optical disk with good electrical characteristics and little distortion in disk shape, even in those cases where a printed layer is provided on the signal readout surface side of the disk.
In order to achieve the above object, an optical disk according to a first aspect of the present invention comprises a transparent disk substrate, a recording layer formed from a translucent reflective film which is layered on pits based on signal information provided on the disk substrate, an optical buffer layer formed from a light transmissive material, which is layered on the recording layer, and a printed layer which is layered on the optical buffer layer and on which is printed visible information, and is constructed so that the visible information of the printed layer is visible from the disk substrate surface of the optical disk through the recording layer and the optical buffer layer, wherein the thickness of the optical buffer layer is an optical distance of between 3 xcexcmxcx9c45 xcexcm.
By providing an optical buffer layer between the recording layer and the printed layer, in the manner described above, and prescribing the thickness of the optical buffer layer as an optical distance of between 3 xcexcmxcx9c45 xcexcm, the scattered and reflected light resulting from light which reaches the printed layer is able to be smoothed, thereby improving the electrical characteristics (jitter). Furthermore, distortion of the disk can be reduced, enabling the manufacture of an optical disk with little curvature and with a good degree of uniformity across the entire disk.
The optical distance described above is the value of the physical distance L multiplied by the refractive index n of the material which forms the optical buffer layer, and is represented by the formula Lxc3x97n.
Furthermore, the optical disk of the present invention has a reflectance of 45%xcx9c55% relative to a reproduction light beam. By prescribing a reflectance relative to the reproduction light beam as 45% or above, as is prescribed in DVD standards, reproduction of optical disks using DVD players can be ensured. Moreover by setting the reflectance relative to the reproduction light beam to 55% or below, the visibility of the visible information on the printed layer can also be ensured.
In an optical disk disclosed in Japanese Unexamined Patent Publication No. 11-39723, because the sputtering time for the formation of an Au translucent reflective film was a relatively short 2.7 seconds, the reflective film was thin and the reflectance was low. Consequently, the reading precision of the information was lowered, making the disk unsuitable for use with DVD.
According to the present invention, the optical buffer layer is preferably formed from a light transmissive material for which the imaginary part k of the complex refractive index satisfies the requirement that kxe2x89xa61.0.
The material for formation of the optical buffer layer should preferably not absorb light, but absorption of light in the wavelength of the readout laser beam is acceptable. Provided the imaginary number portion (absorption coefficient) k of the complex refractive index which represents the light absorption of the material which makes up the optical buffer layer satisfies the requirement that kxe2x89xa61.0, then there are no problems with visibility of the visible information. Materials for which k=0 are transparent with no light absorption properties.
According to the present invention, if the printed layer is constructed from a first printed layer for printing visible information, and a second printed layer for printing a background color for the visible information, then by selecting a suitable background color, the visibility of the visible information can be further improved. In such a case, if the second printed layer is formed by contact printing of a single color, then the printing process is relatively simple.
Furthermore, if the first printed layer and the second printed layer each have a thickness of 2 xcexcmxcx9c30 xcexcm, then not only is the visible information of the printed layer readily visible, but the effect by the printed layer on the mechanical characteristics of the optical disk can also be kept to a minimum.
According to the present invention, the material of the aforementioned translucent reflective film may be selected from the group consisting of Au, Au alloy, Cu, Cu alloy, Si, an alloy incorporating Si as a main component, a compound of Si with either carbon or nitrogen, and a mixture of Si and compound of Si with either carbon or nitrogen.
In order to apply the present invention to a double sided recording reproduction type optical disk, an optical disk in which the optical buffer layer has the type of characteristics according to the first aspect of the invention may be used for at least one of a first optical disk and a second optical disk, with the first optical disk and the second optical disk then being bonded together with the respective substrates thereof facing outwards, thereby providing visible information on only one side of the double sided optical disk. Alternatively optical disks in which the buffer layer has the type of characteristics according to the first aspect of the invention may be used for both a first optical disk and a second optical disk, with the two disks then being bonded together with the printed layers facing each other, thereby providing visible information on both sides of the double sided optical disk. In either case, if an opaque adhesive layer is provided between the first optical disk and the second optical disk to bond the two disks together, then the visibility of the visible information is able to be improved.
A double sided recording reproduction type optical disk of the present invention preferably has a reflectance of between 45%xcx9c55% relative to the reproduction light beam. Furthermore the optical buffer layer of the optical disk is preferably formed from a light transmissive material for which the imaginary part k of the complex refractive index satisfies the requirement that kxe2x89xa61.0. Moreover, the printed layer may be constructed from a first printed layer for printing visible information, and a second printed layer for printing a background color for the visible information, with the second printed layer being formed by single color contact printing. In such cases preferably the first printed layer and the second printed layer each have a thickness of 2 xcexcmxcx9c30 xcexcm. Furthermore, the material of the translucent reflective film may be selected from the group consisting of Au, Au alloy, Cu, Cu alloy, Si, an alloy incorporating Si as a main component, a compound of Si with either carbon or nitrogen, and a mixture of Si and compound of Si with either carbon or nitrogen.